In large-scale machine tools and industrial machines, there are cases where a plurality of servomotors are used to drive axes. FIG. 1 shows a configuration of a conventional servomotor control system. FIG. 1 shows an example of a servomotor control system in which a plurality of (e.g., (M+N)) drive motors (M1-1, M1-2, . . . , M1-M, . . . , M1-(M+N)) are used as the servomotors for driving axes in a machine tool or the like.
The (M+N) drive motors (M1-1, M1-2, . . . , M1-M, . . . , M1-(M+N)) are driven by respective drive amplifiers (SV1-1, SV1-2, . . . , SV1-M, . . . , SV1-(M+N)) each formed of an inverter circuit. Respectively connected to the (M+N) drive amplifiers (SV1-1, SV1-2, . . . , SV1-M, . . . , SV1-(M+N)) are converter circuits (PS-1, PS-2, . . . , PS-M, . . . , PS-(M+N)). The converter circuits convert alternate current (AC) power from an AC power supply (not shown) into direct current (DC) power to supply the drive amplifiers. The drive amplifiers receive DC power from the converter circuit and convert the DC power into AC power to drive the drive motors.
In the above way, in a large-scale machine tool or industrial machine including a plurality of drive motors, the maximum power consumed by the servomotors is large. Accordingly, when the capacity of the installed power supply becomes insufficient, it is necessary to operate the machine by suppressing the output so as not to consume power or enhance the capacity of the installed power supply.
In order to reduce the power consumption for the drive motors, there has been a proposed technique in which a servomotor (buffer motor) equipped with an inertial body is rotated to store power as rotational energy. In this method, when the drive motors consume power, the buffer motor is retarded to supply energy. On the other hand, when the drive motors regenerate electric power, the buffer motor is accelerated to consume energy. For example, there have been known technologies for a pressing machine using a flywheel as motive power to perform a pressing process in which in order to save energy, the flywheel is accelerated and decelerated in accordance with the power consumed at the servomotor axes (Japanese Patent Application Laid-open 2013-71123 (JP 2013-71123 A) and Japanese Patent Application Laid-open 2010-221221 (JP 2010-221221 A)
FIG. 2 shows a configurational diagram of a conventional servomotor control system equipped with buffer motors. This system is the same as that in FIG. 1 in that (M+N) drive motors (M1-1, M1-2, . . . , M1-M, . . . , M1-(M+N)), (M+N) drive amplifiers (SV1-1, SV1-2, . . . , SV1-M, . . . , SV1-(M+N)) and (M+N) converter circuits (PS-1, PS-2, . . . , PS-M, . . . , PS-(M+N)) are provided. The servomotor control system in FIG. 2 includes, in addition to the components shown in FIG. 1, (M+N) buffer motors (M2-1, M2-2, . . . , M2-M, M2-(M+N)) respectively having inertial bodies (IS-1, IS-2, . . . , IS-M, . . . , IS-(M+N)). The (M+N) buffer motors (M2-1, M2-2, . . . , M2-M, . . . , M2-(M+N)) are driven by buffer amplifiers (SV2-1, SV2-2, . . . , SV2-M, . . . , SV2-(M+N)), respectively. The (N+M) buffer amplifiers (SV2-1, SV2-2, . . . , SV2-M, . . . , SV2-(M+N)) are respectively connected to the converter circuits (PS-1, PS-2, . . . , PS-M, . . . , PS-(M+N)). The converter circuit converts AC power from an AC power supply (not shown) into DC power to supply the buffer amplifier. The buffer amplifier receives DC power from the converter circuit and converts the DC power into AC power to drive the buffer motor.
This method makes it possible to reduce the capacity of the installed power supply and the capacity of the converter circuits compared to the case where no buffer motors are used.
However, in the case where there are plural converter circuits for supplying power to drive motors as shown in FIG. 2, plural buffer motors are needed or it is necessary to replace the plural buffer motors, plural buffer amplifiers and plural converter circuits with single integrated units. FIG. 3 shows an example of a servomotor control system in which plural buffer motors, plural buffer amplifiers and plural converter circuits are integrated into respective units. This system includes (M+N) drive motors (M1-1, M1-2, . . . , M1-M, . . . , M1-(M+N)) and (M+N) drive amplifiers (SV1-1, SV1-2, . . . , SV1-M, . . . , SV1-(M+N)), whereas a single converter circuit PS, buffer amplifier SV2 and buffer motor M2 are provided alone. Further, the buffer amplifier SV2 and buffer motor M2 are connected by a single lead wire.
When, as shown in FIG. 2, as many buffer motors as the plural drive motors are used, there is a problem that the number of buffer motors increases with increase of drive motors. On the other hand, when, as shown in FIG. 3, plural buffer motors, plural buffer amplifiers, and plural converter circuits are integrated into single units, a high-capacity converter circuit as well as a large-current inverter circuit is needed to drive the buffer motor. Due to the small number of high-capacity inverter circuits and converter circuits produced, the costs for these is high compared to inverter circuits and converter circuits of general capacities, bringing about a disadvantage.
It is therefore an object of the present invention to provide a servomotor control system that includes a plurality of drive motors and a plurality of inverter circuits for driving the drive motors and that does not use any dedicated high-capacity inverter circuits for driving buffer motors and converter circuits.